Wednesday, January 30, 2008

Under hood Maintenance: Engine oil

Question:

My engine uses about a quart of oil every 1,000 miles. Should I be concerned? Answer:

Not if you plan to sell or trade your vehicle soon. An engine that uses a quart of oil every 1,000 miles is starting to show the effects of wear. The amount of oil it is using is still acceptable, but it will gradually increase as the miles add up. When it reaches the point where it's using a quart every 500 miles or less, it's time for an overhaul. Oil consumption depends primarily on two things: the valve guides and piston rings. If the valve guides are worn, or if there's too much clearance between the valve stems and guides, or if the valve guide seals are worn, cracked, missing, broken or improperly installed, the engine will suck oil down the guides and into the cylinders. The engine may still have good compression, but will use a lot of oil.

An oil consumption problem caused by worn valve guides can usually be cured by a valve job. Knurling or replacing the guides or boring out the guides and installing valves with oversized stems will stop the loss of oil.

Oil can also get past the rings if the rings or cylinders are badly worn or damaged, if the cylinders were not honed properly when the engine was built (or rebuilt), or if the rings were installed improperly.

When a newly-built engine is first started, the rings require a certain amount of time to "seat" or break-in. If the rings fail to seat properly, the engine will use oil. This may be the case if somebody applied the wrong finish to the cylinders, failed to clean and lubricate the cylinders properly before the engine was fired up, or didn't use the proper break-in procedure.

If the rings and/or cylinders are at fault, the engine will have lower than normal compression readings.

In some instances, worn rod bearings, excessive bearing clearances and/or excessive oil pressure can splash too much oil on the cylinders causing oil to get past the rings.

The cure for worn rings and cylinders is to overhaul the engine block. The cylinders have to be refinished and new rings installed to regain good oil control.

Question:

My engine is leaking oil past the valve cover gaskets. I've tried tightening down the valve cover bolts, but it doesn't seem to help. What should I do? Answer:

Bite the bullet and buy a new set of valve cover gaskets. Most cork valve cover gaskets usually cost less than $20 and are fairly easy to install on many engines. You may have to disconnect and remove some plumbing or other accessories to get to the valve covers, but on many engines the job is usually within the capabilities of a do-it-yourselfer. If the valve covers are buried or access is difficult, then let a professional replace the gaskets for you. Tightening the valve cover bolts or screws will rarely stop an oil leak because the gasket is usually cracked, crushed or has lost its natural elasticity. Cork gaskets only last about four to six years before they age harden, become brittle and start to leak. Molded silicone rubber gaskets, on the other hand, (which are used on many late model domestic and import engines) often last the life of the engine. But molded rubber gaskets are a lot more expensive than die cut cork gaskets. That's why cork gaskets have long been used by the vehicle manufacturers.

Some engines do not have gaskets, but instead use a rubbery-glue called "RTV silicone sealer (the "RTV" stands for Room Temperature Vulcanizing). If this is the case, you can remove the valve cover, scrape off all the old RTV, and either apply a fresh bead of RTV silicone sealer to the valve cover flange or head mating surface or install a conventional gasket.

CAUTION: Do not let any pieces of rubber or debris fall into the engine. Also, if you decide to use RTV sealer and your engine has an oxygen sensor (which almost all 1981 and later engines do), make sure the RTV sealer is the "low volatile" variety that is approved for use with oxygen sensors. Some types of RTV give off silicone vapor that can be sucked through the crankcase and contaminate the oxygen sensor.

Question: Is it better to maintain my engine's oil level at the full mark or wait until it reaches the "add" mark to add oil?

Answer: Most vehicle manufacturers say it's okay to wait until the level reaches the add mark to add oil. The add mark is typically one quart below the full mark on the dip stick.Considering that the crankcase capacity on most passenger cars today is only four quarts, running the engine 25% low on oil (one quart) may not be wise. Here's why.

Oil not only lubricates the engine's internal parts, but also helps cool the bearings. The total amount of oil in the engine, therefore, serves as a heat sink to help control heat. Under normal driving conditions, running a quart low probably doesn't make much difference in terms of bearing temperature or overall engine lubrication. But during extremely hot weather, when driving at sustained highway speeds and/or when towing a trailer, running a quart low may increase the risk of accelerated engine wear and/or damage.

The best advice, therefore, is to add oil whenever the dipstick reads low. Don't wait until it is down a full quart. If it needs half a quart, add half a quart to bring it back up to the full mark.

CAUTION: Do not overfill the engine. Adding too much oil can overfill the crankcase. As the crankshaft spins around, it can whip the oil into foam if the level is too high. This, in turn, can cause a drop in oil pressure and loss of lubrication to critical engine parts. Also, too much oil may cause leaks as the extra oil is forced past seals and gaskets.

Follow your vehicle manufacturer's guidelines for the type and viscosity of oil to use in your engine.

Question:

How often should I change my oil? Answer:

Most vehicle manufacturers recommend changing the oil once a year or every 7,500 miles in passenger car and light truck gasoline engines. For diesel engines and turbocharged gasoline engines, the usual recommendation is every 3,000 miles or six months. If you read the fine print, however, you'll discover that the once a year, 7,500 mile oil change is for vehicles that are driven under ideal circumstances. What most of us think of as "normal" driving is actually "severe service" driving. This includes frequent short trips (less than 10 miles, especially during cold weather), stop-and-go city traffic driving, driving in dusty conditions (gravel roads, etc.), and driving at sustained highway speeds during hot weather. For this type of driving, which is actually "severe service: driving, the recommendation is to change the oil every 3,000 miles or six months.

For maximum protection, most oil companies say to change the oil every 3,000 miles or three to six months regardless of what type of driving you do.

A new engine with little or no wear can probably get by on 7,500 mile oil changes. But as an engine accumulates miles, blow by increases. This dumps more unburned fuel into the crankcase which dilutes the oil. This causes the oil to break down. So if the oil isn't changed often enough, you can end up with accelerated wear and all the engine problems that come with it (loss of performance and fuel economy, and increased emissions and oil consumption).

OIL ANALYSIS

Truck fleets often monitor the condition of the oil in their vehicles by having samples analyzed periodically. Oil samples are sent to a laboratory that then analyzes the oil's viscosity and acid content. Oil is then burned in a device called a spectrometer that reveals various impurities in the oil. From all of this, a detailed report is generated that reveals the true condition of the oil.

Oil analysis is a great idea for fleets and trucks that hold a lot of oil. But most consumers would have a hard time justifying the cost. Having an oil sample analyzed typically costs $12 to $20 for the lab work and report. Most quick lube shops charge $16.95 to $19.95 for an oil change. So why spend your money on a report that will probably tell you your oil needs changing? Just change the oil every 3,000 miles and don't worry about it.

Regular oil changes for preventative maintenance are cheap insurance against engine wear, and will always save you money in the long run if you keep a car for more than three or four years. It's very uncommon to see an engine that has been well maintained with regular oil changes develop major bearing, ring, cam or valve problems under 100,000 miles.

WHAT ABOUT THE OIL FILTER?

To reduce the costs of vehicle ownership and maintenance, many car makers say the oil filter only needs to be replaced at every other oil change. Most mechanics will tell you this is false economy.

The oil filters on most engines today have been downsized to save weight, cost and space. The "standard" quart-sized filter that was once common on most engines has been replaced by a pint-sized (or smaller) filter. You don't have to be a rocket scientist to figure out that a smaller filter has less total filtering capacity. Even so, the little filters should be adequate for a 3,000 mile oil change intervals -- but may run out of capacity long before a second oil change at 6,000 or 15,000 miles.

Replacing the oil filter every time the oil is changed, therefore, is highly recommended.

An engine's main line of defense against abrasion and the premature wear it causes is the oil filter. The filter's job is to remove solid contaminants such as dirt, carbon and metal particles from the oil before they can damage bearing, journal and cylinder wall surfaces in the engine. The more dirt and other contaminants the filter can trap and hold, the better.

In today's engines, all the oil that's picked up by the oil pump is routed through the filter before it goes to the crankshaft bearings, cam bearings and valve train. This is called "full-flow" filtration. It's an efficient way of removing contaminants, and it assures only filtered oil is supplied to the engine. In time, though, accumulated dirt and debris trapped by the filter begin to obstruct the flow of oil. The filter should be changed before it reaches this point, which is why the filter needs to be replaced when the oil is changed.

If you wait too long to change the filter, there's a danger that it might become plugged. To prevent this from causing a catastrophic engine failure due to loss of lubrication, oil filters have a built-in safety device called a "bypass valve." When the pressure drop across the filter exceeds a predetermined value (which varies depending on the engine application), the bypass valve opens so oil can continue to flow to the engine. But this allows unfiltered oil to enter the engine. Any contaminants that find their way into the crankcase will be pumped through the engine and accelerate wear.

FILTER REPLACEMENT

If you do your own oil changes, make sure you get the correct filter for your engine. Follow the filter manufacturer's listings in its catalog. Many filters that look the same on the outside have different internal valving. Many overhead cam engines, for example, require an "anti-drainback" valve in the filter to prevent oil from draining out of the filter when the engine is shut off. This allows oil pressure to reach critical engine parts more quickly when the engine is restarted. Filters that are mounted sideways on the engine typically require an anti-drainback valve.

CAUTION: The threads on a spin-on filter must also be the correct diameter and thread pitch (SAE or metric) for your engine. If you install a filter with SAE threads on an engine that requires metric threads (or vice versa), you can damage the threads that hold the oil filter in place. Mismatched threads can also allow the filter to work loose, which causes a sudden loss of oil pressure that may ruin your engine!

CHANGING YOUR OWN OIL

Some people say it's best to change the oil when the oil is hot (like right after driving), while others say it makes no difference. CAUTION: Hot oil is thinner and runs out faster but can also burn you if you're not careful. In any event, avoid unnecessary skin contact with oil because oil is a suspected carcinogen (causes cancer).

Changing the oil when it is cold may take a bit longer because the oil will drain more slowly from the engine, but there's no danger of being burned. Also, most of the oil will have drained down into the oil pan when the engine has sat for a period of time, which means you'll actually get a little more of the old oil out of the engine than if you attempt to drain it while it is still hot.

Used motor oil should be disposed of properly. The Environmental Protection Agency does not consider used motor oil to be a hazardous chemical, but it can foul ground water and does contain traces of lead. The best way to dispose of used motor oil is to take it to a service station, quick lube shop, parts store or other facility for recycling. Your old oil will either be rerefined into other lubricants or petroleum products, or burned as fuel.

Do not dump used motor oil on the ground, down a drain, into a storm sewer or place it in the trash. Many landfills will not accept used motor oil even if it is in a sealed container because it will eventually leak out into the ground. If you can't find an environmentally-acceptable way to dispose of the stuff, maybe you shouldn't be changing your own oil. Service facilities that do oil changes all have storage tanks and recycling programs to dispose of used oil.

Question: What type of motor oil is best for my engine?

Answer: The type specified by the vehicle manufacturer in your owner's manual. For most passenger car and light truck gasoline engines today, it's any oil that meets the American Petroleum Institutes "SH" rating.

As for the viscosity of oil to use, most new engines today require a multiviscosity 5W-30 oil for all-round driving. The lighter 5W-30 oils contain friction reducing additives that help improve fuel economy, and also allow the oil to quickly reach critical upper valvetrain components when a cold engine is first started. Most engine wear occurs immediately after a cold start, so it's important to have oil that is thin enough to circulate easily -- especially at cold temperatures.

For older engines and ones that are driven at sustained highways speeds during hot weather, 10W-30 or 10W-40 is a good choice. Heavier multiviscosity oils such as 20W-40 are for high rpm, high-load applications primarily and are not recommended for cold weather driving.

Straight weight 30W and 40W oils aren't very popular anymore, but some diehards insist on using them. They say the thicker oil holds up better under high temperature (which it does), increases oil pressure and reduces oil consumption in high mileage engines. But straight 30W and 40W oils are too thick for cold weather and may make an engine hard to start. They may also be too thick to provide adequate start-up lubrication to critical upper valvetrain components during cold weather. So switching to a straight 20W oil would be necessary for cold weather driving. Straight 10W oil can also improve cold starting, but is very thin and should only be used in sub-zero climates. A multiviscosity 10W-30 or 10W-40 will provide the same cold starting benefits of a 10W oil and the high temperature protection of a 30W or 40W oil.

SYNTHETIC OILS

For the ultimate in high temperature protection, durability and all-round performance, synthetic oils are the way to go. Unfortunately, most synthetic oils cost up to three times as much as ordinary petroleum-based oils. They cost more because synthetics are manmade rather than refined from petroleum. But this improves their performance in virtually every aspect:

Superior temperature resistance. Synthetics can safely handle higher operating temperatures without oxidizing (burning) or breaking down. The upper limit for most mineral based oils is about 250 to 300 degrees F. Synthetics can take up to 450 degrees F. or higher. This makes synthetics well-suited for turbo applications as well as high rpm and high output engine applications.

Better engine performance. Synthetics tend to be more slippery than their petroleum-based counterparts, which improves fuel economy, cuts frictional horsepower losses and helps the engine run cooler. The difference isn't great, but it can make a noticeable difference.

Longer oil change intervals. Because synthetics resist oxidation and viscosity breakdown better than ordinary motor oils, some suppliers say oil change intervals can be safely extended -- in some cases stretched to as much as 25,000 miles. Such claims are justified by the fact that synthetics don't break down or sludge up as fast as ordinary mineral-based oils do in use.

CAUTION: For vehicles under warranty, extending the normal change interval is not recommended because failing to follow the OEM's maintenance schedule can void your warranty.

Synthetics are available in the same grades as ordinary motor oils (5W-30, 5W-20 and 10W-30) as well as "extended" grades such as 15W-50 and even 5W-50.

There are also lower-cost synthetic "blends" that combine synthetic and petroleum-based oils in the same container. But you can do your own blend to save money by simply substituting a quart or two of synthetic oil for conventional oil when you change oil. Synthetics are compatible with conventional motor oils.

Answer: You should still change your oil and filter periodically to get rid of contaminants and sludge that build up in the crankcase. Just because your engine uses oil doesn't mean the oil that's in it stays clean. It doesn't. It gunks up just as fast as the oil in an engine that doesn't burn oil. In fact, it probably gunks up faster because of increased blow by due to the worn pistons and rings.

If you never change the oil, sooner or later the filter will plug up. And once that happens, you've lost all protection against dirt and abrasives. Before long, the bearings will become worn and you'll hear the rods knocking. Keep driving and something will eventually let go. End of engine. End of story.